1. Field of the Invention
This invention relates to array antennas and is particularly concerned with providing time delay steering to array antenna elements.
2. Description of the Prior Art
The antenna in radar systems and in many communications systems must face the microwave source to achieve the necessary signal to noise ratio, or to provide directional information. A parabolic dish with a receiver element at the focus is convenient to use for steering an antenna in a fixed direction. Moving dishes can be employed for moving sources or for radar systems, but moving dishes do not have the ability to change directions rapidly. Phased arrays may be used to rapidly change direction, however, such arrays are very complex and tend to be expensive. Furthermore, because the steering is determined by the element's spacing along the array, the relative phase shift imposed in the element signal and the frequency, phased arrays may only operate over a very limited bandwidth.
An array antenna consists of a group of antenna elements spaced apart usually at regular intervals to form an array. By either controlling the relative timing or the phasing of the common signal at the receiving elements, the antenna may be steered towards the microwave source.
The most common means of steering an antenna array is to control the relative phase of the signal at the elements. For the case of a flat antenna array, if a beam is pointed toward the array in the boresight direction, which is the direction perpendicular to the plane of the array all the elements will receive the signal in unison. If the beam is pointed (received) at some angle from the boresight direction, the signal is received along the face of the array with a linearly increasing phase shift. Each element is equipped with a phase shifter so that when a signal is received with an increasing phase shift, the phase at each element may be reconciled so that the total signal from each of the elements is in phase. The phase shifters may be "steered" so that a signal that arrives from a selected direction will be in phase.
Controlling the relative phase of each of the antenna elements requires that each element contain a phase shifting device and that an electronic control system be used to control the phase of each of the elements. However, the wide scale use of phased arrays has been limited by the high cost of their complex circuitry. Furthermore, if the phase shifting circuit is adjusted to steer the antenna in a particular direction, this setting will only be valid for a particular frequency. Therefore, known phase shifting techniques impose a limit on the frequency range of operation.
The use of fiber optics in communication systems is known. The optical fiber is usually used to connect two distant points so that communications can be conducted. For the conveyance of signals, commercially available laser unit is used to generate an optical signal. The optical signal travels through the optical fiber to where it is converted to a microwave signal by an optical detector and a microwave amplifier, which are commercially available. Fiber optic media offer the advantage of exceptional bandwidth as compared with other traditional transmission means such as copper wire.
Optical techniques have been suggested to control array elements. Schemes have been proposed to use a selection of optical fibers with lengths arranged in a binary or quadratic sequence and to switch in a series string combination to achieve a desired timing. This would result in a very complex control scheme employing thousands of optical fibers and optical switches for even the simplest phased array.